The present embodiments relate generally to water heaters.
In one application, water heaters are used on aircraft. One factor complicating the design of a water heater unit to be used on an aircraft is the limited space available for locating the unit. For example, on some aircraft water heater units are located under sinks or in lavatories where space is confined. As a result, current aircraft water heater units are compact, providing small quantities of warm water within a relatively small fixed temperature range.
These current aircraft water heater units provide small quantities of warm water within a small temperature range not only because of their size, but also because these units utilize fixed (i.e. static) orifice type mixing valves. The water heater unit receives cold water inside a vessel, heats and stores this water inside the vessel, and delivers the heated water to the fixed mixing valve. At the fixed mixing valve, the heated water is mixed with cold water to create a water mixture which is at a set mixed water outlet temperature. However, the fixed mixing valve is only capable of mixing a constant (i.e. unchanging) amount of cold water with the heated water from the unit. This inability to change the ratio of cold water mixed with the heated water is significant for two reasons. First, the water in the vessel cannot be heated to relatively higher temperatures outside of the fixed temperature range without having to install a different fixed mixing valve. However, the fixed mixing valve is typically welded internal to the vessel, the ends of which are welded on, making installing a different fixed mixing valve prohibitively difficult. Second, water is generally heated and stored inside the unit's vessel at a temperature only slightly above the mixed water outlet temperature. Therefore, using a fixed mixing valve confines the water heater unit to either producing water at an elevated temperature for a short period of time (by installing a different fixed mixing valve), or producing water at a lower temperature for a slightly longer period of time—the unit cannot both heat water to an elevated temperature and provide larger quantities of warm water. Consequently, current aircraft water heater units generally can be used only for a single application due to the limited hot water capacity of these units.
Another type of water heating technology generally used in various applications is demand-type, or instant, water heating. Demand-type water heaters provide warm water only as warm water is needed, and do not utilize a vessel to store heated water. Although a demand-type water heater is capable of supplying larger quantities of warm water and can supply water at an elevated temperature, demand-type water heaters necessitate extremely high power requirements. The high power requirement for a demand-type water heater is typically not available in numerous applications, including aircraft applications.